The UK wind industry is scrambling to respond to a High Court ruling that affirmed the legaltiy of conditions placed on the Den Brook wind farm near Devon, limiting ampltude modulation of wind turbine noise to a level that could be very hard to comply with. After years of pooh-poohing the reports of neighbors who said that the pulsing quality of the turbine noise made it especially hard to live with, including a much-criticized study a few years back that found nearly no AM at UK wind farms, Renewable UK (formerly the British Wind Energy Association) is fast-tracking a far-reaching study of AM, which they hope to complete in just seven months.

The new study, funded by Renewable UK (a trade organization of wind industry companies), aims to develop better models for predicting AM, including assessment of the effects of high turbulance and closely spaced turbines, as well as noise predictions both nearby and at a distance. In addition, they aim to develop a listening test that could inform a possible penalty-assessment approach to dealing with AM noise when it does occur; such an approach, common in many regulations, forces the overall noise level to be lower when AM is present.

After years of claimng there is no need to assess or regulate AM, it appears that the industry has now found itself sufferering the consequences of denying the problem. Instead of working to create regulations that take the issue seriously (whether or not it is common), the industry is now vulnerable to being out of compliance when AM does occur.

The recent ruling unfolded along just these lines. The wind developer claimed noise would be inaudible or at least not problematic, while local resident Mike Hulme was unconvinced and wanted to be sure that if AM did occur, there would be consequences for the wind farm. His acoustical consultant Mike Stigwood told the Noise Bulletin: “I devised an excess amplitude modulation condition based on my findings and measurements at other wind farms that was worded simply and made an exceedence a breach. It was a simple stand-alone condition.” In an earlier round of litigation on these conditions, the developers proposd a penalty approach to dealing with non-compliance (thus seemingly implying that AM could occur), but the Inspector who wrote the rules did not incorporate their proposal, because he felt the proposal lacked necessary detail to apply effectively.

While the High Court ruling denied the appeal’s goal of stopping the Den Brook wind farm from proceeding, it affirmed the validity of the AM condition and stressed that the wind farm must comply with the rules as written, which are very stingent: whenever sound levels are over 28dB, turbine noise (measured in very short time intervals) can’t vary by more than 3dB. To avoid penalizing random momentary fluctuations, the AM provision applies only when this pulsing of sound occurs more than five times in two minutes, and for at least six minutes in any hour.

While ruling that the condition as written was valid, the Court said that there was no provision in the ordinance that would allow any sort of penalty or other way of dealing with non-compliance with the AM limit, short of shutting down or changing operations so as to remove the pulsing sound. It’s likely that this High Court ruling will provide precedent and justification for the development of ordinances that do address Amplitude Modulation as a particular quality of wind turbine sound, and that future ordinances will be developed with a penalty scheme to minimize the negative effects of this pulsing quality of wind turbines, by requiring them to be quieter when AM is present; in practice, this is likely to mean that wind farms will need to be built a bit farther from homes, so that their noise is quieter all the time, leaving room for AM factor to be added without breaking the noise limits.

For more, see this article in The Environmentalist, a leading UK magazine, or read the High Court ruling here. Also fo note, the June 2011 edition of Noise Bulletin includes an in-depth article on the court case, along with a good summary of the Wind Turbine Noise 2011 conference, including a sidebar introducing the industry-funded AM research program; Noise Bulletin is not viewable online, but free sample issues and trial subscriptions are available on their website.

Mike, we watched Wind Farm Wars with horror. Horror that our country is being blighted by these hideously inefficient monsters and Horror at the long, expensive and traumatic process that you and the other residents had to go through.
Your persistence on the subject of AM may well be the saviour of many of us and we are truly grateful for your efforts.

i find the vehemence with which certain groups of people oppose wind farms staggering,the future of our countries power supply lies with wind farms and probably nucleur power if we are to avoid reliance on CO2 emitting power genration. I live in an area with approximately 30 turbines within 3 miles of my home with the closest being approx 400 metres and do not find to be an issue. Whilst the pulse or swoosh of the turbine blades is audible on occasion the sound of this is often muffled by a cow or a passing tractor, the people who have issues with this are perhaps focusing on this subject in a unhealthily obsessive way as these noises just in my opinion form part of the tapestry of sound in a rural “working” farm environment

Chris, your experience living near or within a wind farm is one that is shared by many others in working landscapes. This is one of the paradoxes of coming up with a society-wide approach to wind development: it’s clear that in places like Iowa and Texas, people are just not that bothered by audible wind turbine noise, while in Wisconsin, Ontario, and Maine many more residents feel like these moderate noise levels are an intolerable intrusion. Much has to do with expectations, but I don’t know that the answer is to assume that everyone should be expected to have the same expectations. Different communities DO have different relationships with their landscape and soundscape; working landscapes are meant to be worked, with as you note, the tapestry of sound including many machines at a distance, while some rural areas are inhabited by more people who specifically value the ambient natural soundscape or the sense of being “away from it all”.

We also have to bear in mind the natural spectrum of noise sensitivity in the population; to someone who’s pretty noise tolerant (as you may be, along with about half the general population), it can appear that others are hypersensitive or obsessing on noise; likewise, for someone who’s noise sensitive (as 20% of the population is), it’s hard to imagine how you would not be bothered by that turbine 400m away. The noise sensitive account for the folks who have a hard time as soon as turbines are audible; the other 30% of moderately noise sensitive folks are the ones who start to pipe up when the turbine noise rises to 10 or 20dB over ambient, hitting 45-50dB in their yards. Meanwhile, a big chunk of their neighbors, those who are noise tolerant, say, “what’s the big deal?”

All this is why we really do need to find answers that work for the full range of people, not just for those who are happy in a working landscape and are noise tolerant. It’s not hard to find wind farm sites far enough from homes or to establish a closer-to-ambient noise limit, as Oregon has done. Likewise, even if we adopt a more protective noise limit, we can also allow for easements to be sold by neighbors like you who are not bothered by the occasional moderate noise. The vehemence you note is definitely based on fear that worst-case events elsewhere will be repeated in one’s own community; these fears are likely overblown for all but those who would live the closest; if we can remove the pressure that very close siting creates, then the fears and vehemence are likely to die down.

We are preparing for a court briefing on a planned wind turbine and one of the issues is amplitude modulation. The investors say it diminishes beyond 300-500 m and the environmental authorities believe them.

Do any of you know a peer-reviewed, published study stating the distance within which AM is still audible?

FYI: amplitude modulation (blade swish) is very often clearly audible and present on recordings well beyond 500m, to at least 1km. By 1500m it has generally faded from clear audibility, with low-frequency rumbles the main sound heard, albeit (usually) faintly.

There has just been 4 396ft direct drive Enercon turbines erected just less than a mile (1.465km)from my house (despite huge community objections & rejection by the local council which was then over-ruled by central government), in a rural area with extremely low dB readings at night; at times lower than 20dB. I was extremely worried about the prospect of noise from this wind farm. The turbines are not running at full capacity yet and so far they haven’t been too bad but we can most definitely hear the ‘whoosh’ ‘whoosh’ AM sound from them when the wind is in certain directions, unfortunately we are downwind of them and the west wind blows from that direction 90% of the time. The energy company have set up sound monitoring equipment in our garden and up to press have been ‘concerned’ about our worries but it is plain to see that unless there is a mechanical problem with the turbines and they are not running to spec, the energy company will not do anything about reducing noise levels round our property if they fall within the limits. It really is time all these wind farms were stopped. The cost to erect them and then the annual payment made to the land owner (£13,000 per turbine per year) and the lack of electricity generated from them simply does not make them cost effective. The government should LISTEN, for once, to the majority of public opinions – we don’t want them!

Often environmental noise is regulated based on land use, with residences being among the most noise-sensitive land uses. Residents in rural areas often think they are entitled to a quieter environment. However, under many environmental noise laws they are not considered more noise-sensitive than urban residents.

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